The present invention relates generally to electrostatographic reproduction machines, and more particularly to image to sheet registration in such a machine.
Generally, the process of electrostatographic production or reproduction of an image on a sheet of material is initiated by exposing a light image of an original image document onto a substantially uniformly charged photoreceptive member. The original image document may be manually placed, in registration, at an exposure station for such exposure, or it may be fed automatically by an automatic document-handling device, also in registration, to the exposure station. Exposing the light image onto the charged photoreceptive member discharges areas of a photoconductive surface thereof corresponding to non-image areas in the original document, while maintaining the charge in image areas, thereby creating an electrostatic latent image of the image of the original document on the photoreceptive member.
Thereafter, developing material including charged toner particles is deposited onto the photoreceptive member such that the charged toner particles are attracted to the image areas on the photoconductive surface to develop the electrostatic latent image into a visible image. This developed image is then transferred from the photoreceptive member, either directly or after an intermediate transfer step, to an image receiving support substrate, such as a copy sheet of paper, thus creating a toner image on the support substrate corresponding to the original image of the original document. The image receiving support substrate, such as a copy sheet of paper, typically is fed automatically from a supply source, and in timed registration, to an image transfer station for receiving the toner image as such. Subsequently, the transferred image is typically fused and affixed to the image support substrate to form a permanent image thereon. In a final step, the photoconductive surface of the photoreceptive member is cleaned to remove any residual developing material thereon in preparation for successive imaging cycles.
Sheet handling devices are commonly used in printing systems, and particularly in electrostatographic reproduction machines of the type described hereinabove, for transporting and registering document and copy substrate sheets to predetermined locations required for accomplishing the printing process. Such sheet handling devices are generally referred to in two categories: document handlers, which are used to transport image bearing sheets; and copy substrate sheet handlers, which transport blank page sheets of material for receiving toner images. Printers, duplicators and copiers commonly employ both types of sheet handling devices to transport sheets to and from an image reproduction or imaging subsystem. As pointed out above, such subsystems or stations include the exposure or image input scanning station, and the toner image transfer station. Image input devices, which include scanners, optical character readers and the like, also employ sheet handling devices of the type to which this invention relates.
In systems employing such sheet handling devices, maintaining proper alignment of the image support sheet along the transport path thereof so as to inhibit skew or misalignment of the sheet being transported is an important function required for acceptable performance. For example, it is important to deskew or inhibit skew in a transported document sheet in a typical electrostatographic reproduction machine employing an automatic document handler device. In such machines, the automatic document handler device automatically transports or feeds a document sheet from a stack thereof to a registered position at the exposure station. As such, it is important to deskew or inhibit skew in the transported document sheet so as to provide proper registration of the image on the document sheet to an imaging frame of the photoconductive member which is then at a fixed position at the exposure station.
Similarly, it is important in a copy sheet handling device of the machine, to deskew or inhibit the skew of a transported copy sheet during image transfer, in order to provide proper registration of the copy sheet to the toner image on the photoconductive member. Failure to properly control skewing and registration of input documents in a document handler, or in copy sheets being handled by a copy sheet handling device, will result in the image produced being misaligned relative to the edges of the copy sheet, and hence being of poor quality. In addition, failure to properly deskew a document or copy sheet can cause jams and other similar paper transport problems. Thus, in sheet transport devices, such as document feeders and automatic or semiautomatic document handlers, as well as in copy sheet transport devices, proper control of skew and registration of sheets being handled, are important and essential system requirements.
Adequate image to paper registration and skew performance are two attributes that are difficult to design into a printing system. Generally, high performance printers allow for offline measurement of registration and/or skew performance. This measurement is performed in the factory or at installation of a printing system at a customer site. Typically a service technician visually examines registration marks on a printed page and then makes any necessary machine adjustments. This human intervention to perform registration and/or skew measurement is both time-consuming and costly to the customer. A method that provides for machine measurement of registration and/or skew without assistance would present significant cost and reliability improvements.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
U.S. Pat. No. 5,555,084 to Vetromile et al. discloses an apparatus for registration of a sheet with a developed image on a moving surface. The apparatus includes a transfer station, detector, registration controller, and a registration transport. The detector, located at the transfer station, includes a pair of lead edge sensors and a side edge sensor. The lead edge sensors detect the lead edge of a sheet of paper and provide skew information to a registration controller until the side edge sensor signals the registration controller of an edge-registered condition. Until the signal from the side edge sensor is received, differential motors correct the skew by driving a pair of rolls, which move the sheet of paper to correct the paper skew.
U.S. Pat. No. 4,971,304 to Loftus discloses an edgeless sheet registration system suitable for an electrophotographic printing machine. The registration system transports a sheet to a transfer zone, wherein the edges of the sheet are synchronized with an image developed on a photoconductive belt. Sheet registration is accomplished in the process and lateral directions, as well as for skew position. The sheet velocity is also matched to the velocity of the belt. In operation, two separate motors drive the sheet non-differentially in the process direction. The sheet is driven until it reaches two optical sensors, which detect passage of selected sheet portions thereby. Signals from the sensors are communicated to a controller, wherein the time difference between passage of the selected sheet portions is used to compensate the random skew. Compensation is achieved by the controller driving the motors differentially, so as to guide the sheet into a preselected skew magnitude. The motors continue to run differentially to compensate for the induced skew until a side edge of the sheet is detected by a third sensor. Detection by the third sensor establishes registration in the lateral direction. Thereafter, the motors again run in a non-differential mode to drive the sheet in the process direction. A fourth sensor located downstream, along the path of travel, is provided to detect the time of passage of the registered sheet thereby.
U.S. Pat. No. 4,416,534 to Kluger discloses a registration method and apparatus for a variable pitch copier. Registration is accomplished through achieving a speed and position match between a copy sheet and an image on a photoconductor as the copy sheet approaches an image transfer station. The speed and position of both image and copy sheet are monitored and updated by a programmed microprocessor. Controlled accelerations and brakings of a copy sheet drive motor under microprocessor control first achieve registration and then maintain that registration as the image transfer occurs. The disclosed registration method automatically adjusts for variable spacings between successive images about the periphery of the photoconductor to accommodate various image sizes.